There is a substantial body of art describing various materials and techniques for using biocompatible implants in the human body. Some of the more important needs are for hip joints, knee and spine reconstruction and shoulder joints. These implants are usually metallic since they are load bearing structures and require relatively high strengths. To insure proper fixation to the bone, a porous metal coating is applied to the implant surface, and is positioned such that it is in contact to the bone. This is to promote bone growth into and through the porous coating to insure a strong bond between the bone and the metallic implant. This coating requires a high degree of porosity, typically greater than 50% and as high as 70-80% with open connective pore sizes varying from 100μ to 500μ. These implants must have significant compression strength to resist loads that these joints can experience. The modulus must also be closely matched to that of the bone to avoid stress degradation of the adjoining tissue.
In addition to the use of tantalum fibers for bone growth, repair and attachment of the implant, it can also be used effectively as a scaffold for soft tissue growth and can provide either a permanent or temporary support to the damage tissue/organ until functionalities are restored. Regardless of whether it's soft or hard tissue repair/replacement, all biomaterial will exhibit specific interactions with cells that will lead to stereotyped responses. The ideal choice for any particular material and morphology will depend on various factors, such are osteoinduction, Osteoconduction, angiogenesis, growth rates of cells and degradation rate of the material in case of temporary scaffolds.
Tissue engineering is a multidisciplinary subject combining the principles of engineering, biology and chemistry to restore the functionality of damaged tissue/organ through repair or regeneration. The material used in tissue engineering or as a tissue scaffold can either be naturally derived or synthetic. Further classification can be made based on the nature of application such as permanent or temporary. A temporary structure is expected to provide the necessary support and assist in cell/tissue growth until the tissue/cell regains original shape and strength. These types of scaffolds are useful especially in case of young patients where the growth rate of tissues are higher and the use of an artificial organ to store functionality is not desired. However, in the case of older patients, temporary scaffolds fail to meet the requirements in most cases. These include poor mechanical strength, mismatch between the growth rate of tissues and the degradation rate of said scaffold. Thus older patients need to have a stronger scaffold, which can either be permanent or have a very low degradation rate. Most of the work on scaffolding has been done on temporary scaffolds owing to the immediate advantages realized of the materials used and the ease of processing. Despite early success, tissue engineers have faced challenges in repairing/replacing tissues that serve predominantly biomechanical roles in the body. In fact, the properties of these tissues are critical to their proper function in vivo. In order for tissue engineers to effectively replace these load-bearing structures, they must address a number of significant questions on the interactions of engineered constructs with mechanical forces both in vivo and in vitro.
Once implanted in the body, engineered constructs of cells and matrices will be subjected to a complex biomechanical environment, consisting of time-varying changes in stresses, strains, fluid pressure, fluid flow and cellular deformation behavior. It is now well accepted that these various physical factors have the capability to influence the biological activity of normal tissues and therefore may plays an important role in the success or failure of engineered grafts. In this regard, it is important to characterize the diverse array of physical signals that engineered cells experience in vivo as well as their biological response to such potential stimuli. This information may provide an insight into the long-term capabilities of engineered constructs to maintain the proper cellular phenotype.
Significant advances have been made over the last four decades in the use of artificial bone implants. Various materials ranging from metallic, ceramic and polymeric materials have been used in artificial implants especially in the field of orthopedics. Stainless steel (surgical grade) was widely used in orthopedics and dentistry applications owing to its corrosion resistance. However later developments included the use of Co—Cr and Ti alloys owing to biocompatibilities issues and bioinertness. Currently Ti alloys and Co—Cr alloys are the most widely used in joint prostheses and other biomedical applications such as dentistry and cardio-vascular applications. Despite the advantages of materials such as Ti and Co—Cr and their alloys in terms of biocompatibility and bio inertness, reports indicated failure due to wear and wear assisted corrosion. Ceramics was a good alternative to metallic implants but they too had their limitation in their usage. One of the biggest disadvantages of using metals and ceramics in implants was the difference in modulus compared to the natural bone. (The modulus of articular cartilage varies from 0.001-0.1 GPa while that of hard bone varies from 7-30 GPa). Typical modulus values of most of the ceramic and metallic implants used lies above 70 GPa. This results in stress shielding effect on bones and tissues which otherwise is useful in keeping the tissue/bone functional. Moreover rejection by the host tissue especially when toxic ions in the alloy, such as Vanadium in Ti alloy, are eluted causes discomfort in patients necessitating revisional operations to be performed. Polymers have modulus within the range of 0.001-0.1 GPa and have been used in medicine for applications which range from artificial implants, i.e., acetabular cup, to drug delivery systems owing to the advantages of being chemically inert, biodegradability and possessing properties, which lies close to the cartilage properties. With the developments in the use of artificial implants there were growing concerns on the biocompatibility of the materials used for artificial implants and the immuno-rejection by the host cells. This led to the research on the repair and regeneration of damaged organs and tissues, which started in 1980 with use of autologuous (use of grafts from same species) skin grafts. Thereafter the field of tissue engineering has seen rapid developments from the use of synthetic materials to naturally derived material that includes use of autografts, allografts and xenografts for repair or regeneration of tissues.
Surface area and surface terrain or topography is one of the important factors governing cell adhesion and proliferation, and there have been many studies carried out in recent times to investigate the suitability of materials such as spider webs and cover slips, fish scales, plasma clots, and glass fibers. Silk fibers also have been used extensively in surgical applications such as for sutures and artificial blood vessels. Cell adhesion to materials is mediated by cell-surface receptors, interacting with cell adhesion proteins bound to the material surface. In aiming to promote receptor medicated cell adhesion the surface should mimic the extracellular matrix (ECM). ECM proteins, which are known to have the capacity to regulate such cell behaviors as adhesion, spreading, growth, and migration, have been studied extensively to enhance cell-material interactions for both in vivo and in vitro applications. However, the effects observed for a given protein have been found to vary substantially depending on the nature of the underlying substrate and the method of immobilization. In biomaterial research there is a strong interest in new materials especially for metals, which combine the required mechanical properties with improved biocompatibility for bone implants and soft tissue repair and replacement.
The foregoing discussion of the prior art derives in large part from an article by Yarlagadda, et al. entitled Recent Advances and Current Developments in Tissue Scaffolding, published in Bio-Medical Materials and Engineering 15(3), pp. 159-177 (2005).
As noted supra, high surface area and surface terrain or topography is one of the important factors governing cell adhesion and proliferation. The greater the relative surface area, i.e., the greater the specific surface of the material the greater cell adhesion and proliferation. Prior attempts to increase specific surface area of materials used for implants generally involve making the material used for forming the implant as small as possible. However, making the material as small as possible adds significantly to material manufacturing cost, and also complicates handling.
See U.S. Pat. No. 5,030,233 to Ducheyne, who discloses a mesh sheet material for surgical implant formed of metal fibers having a fiber length of about 2 mm to 50 mm, and having a fiber diameter of about 20 to about 200 um. According to the '233 patent if the fiber length is more than about 50 mm, manufacturing becomes difficult. In particular, for fiber lengths in excess of about 50 mm, sieving the fibers becomes impractical if not impossible. If the diameter of the fibers is less than about 20 microns, it is difficult to maintain the average pore size of at least 150 μm needed to assure ingrowth of bony tissue. If the fiber diameter is greater than about 200 μm, the flexibility and deformability become insufficient.
See also U.S. Pat. No. 4,983,184 to Steinemann which describes the use of metallic fibers, circular in cross-section, and having a thickness of 5 to 20 micrometers, formed of titanium alloy, bundled together as 200 to 1000, or even up to 3000 fibers, for forming an alloplastic reinforcing material for soft tissue. More particularly, Steinemann teaches titanium and titanium alloys for producing artificial soft tissue components and/or for reinforcing natural soft tissue components comprising elongate titanium or titanium alloy wires of diameter 5 to 20 micron diameter, bundled together for use as an artificial soft tissue component and reinforcement for a soft tissue component in a human or animal. Pure tantalum metal is known to have excellent bio-compatible properties and has for many years been used in the medical field. Indeed, a paper by Bobyn, Stackpool, Hacking, Tanzer and Krygier in the Journal of Bone & Joint Surgery (Br), Vol. 81-B, No. 5, September, 1999, and in U.S. Pat. No. 5,282,861 to Kaplan describes the use of porous tantalum bio material for use to promote bone growth and adhesion of the metallic implants, which material currently is marketed by the Zimmer Corp.